1. Field of the Invention
The present invention relates to an optical device, more particularly, a backlight device for use in a liquid crystal display (LCD) or the like.
2. Description of the Related Technology
Since an LCD is not self-luminous, such an LCD basically uses a surface light source device arranged at the back side of the LCD, so called a “backlight.” The amount of light emitted from such a surface light source device is finite. For this reason, in order to improve the screen brightness of the LCD, it is necessary to achieve an enhancement in utilization efficiency of light. To this end, attempts to enhance the utilization efficiency of light have been made in such a way of enhancing the brightness of the LCD with various optical elements disposed between the surface light source device and a liquid crystal panel, or the like.
FIG. 12 shows a conventional liquid crystal display 10. As shown in FIG. 12, the liquid crystal display 10 mainly comprises a light source 20, a liquid crystal panel 30, and an optical processing unit 40 interposed between the light source 20 and the liquid crystal panel 30. The light source 20 comprises a casing 24 having a reflective layer coated on an inner surface thereof, and a plurality of rod-shaped lamps 22, each of which is composed of a cold cathode fluorescent lamp (CCFL). The liquid crystal panel 30 comprises a liquid crystal cell 32, phase difference plates 34A and 24B having the liquid crystal cell 32 interposed therebetween, and polarization plates 36A and 36b having the phase difference plates 34A and 24B interposed therebetween. The optical processing unit 40 comprises a diffusion plate 42, a diffusion sheet 44, a prism sheet 46, and a brightness enhancing film 48 sequentially stacked from the light source 20 towards the liquid crystal panel 30.
The liquid crystal display performs a screen display function using a liquid crystal display diode as a shutter for light, and comprises the polarization plates 36A and 36B for the shutter function. Generally, since the polarization plates 36A and 36B absorb polarized components of light traveling in a direction of orthogonal to a transmission axis, utilization efficiency of light cannot enhance to 50% in theory.
In order to prevent loss in utilization efficiency of light while increasing the brightness, liquid crystal display is provided with a brightness enhancing film 48 which comprises a reflective polarization plate. The reflective polarization plate reflects the polarized component of light traveling in the direction of orthogonal to the transmission axis, and turns it back to the light source 20, thereby enhancing the utilization efficiency of light. One example of the reflective polarization plate is disclosed in Japanese Patent Laid-open No. 2002-303731.
A conventional polarization film is designed to maximize the utilization efficiency of light at the inherent wavelength of light. Although a visible light has a wavelength in the range of 400˜700 nm, the conventional polarization film has a problem in that sufficient utilization effect of light cannot be achieved over the entire range of the visible wavelength band as described above.